These studies combine biochemical and physiologic approaches towards understanding the biology and the regulation of lysyl-derived collagen cross-links in human cardiovascular structures. Using the Ivalon sponge-implant model in rats for studying injury and repair, we established the profiles of borohydride-reducible collagen cross-links as collagen fibers thicken and bundles form. Three post-translational steps involving lysyl ad hydroxylysyl residues in the organization of collagen into fibers and bundles were recognized. The third and final step seems to involve the formation of non-glycosylated dehydrodihydroxylysinonorleucine (DHLNL). Additionally, the amounts of borohydride-reducible DHLNL appear to be inversely related to the susceptibility of collagen to enzymatic degradation, and the amount of reducible DHLNL per unit of newly-formed collagen is greater in the male than in the female rat. Another collagen cross-link that may affect the age-related stability and tensile strength of collagen is hydroxyaldolhistidine (HAH). HAH is a tri-functional cross-link in that it links three collagen molecules in the fibril. Currently, we are isolating for amino acid sequencing the peptide fraction of collagen that is cross-linked by non-glycosylated DHLNL (in developing collagen of sponge-implant) and that cross-linked by HAH (in mature collagen of bovine skin). This information should locate precisely these two potentially-key cross-links along the chain of the collagen molecule. Ultimately, the results obtained from these model systems will be transferred to vascular tissues. Success in this transformation should provide a rational basis for understanding and managing some of the age-related complications of cardiac valvular and arterial tissues in man.